Radiation sensitive silver halide emulsions containing one or a combination of chloride, bromide and iodide ions have been long recognized to be useful in photography. Each halide ion selection is known to impart particular photographic advantages. By a wide margin the most commonly employed photographic emulsions are silver bromide and silver bromoiodide emulsions. Although known and used for many years for selected photographic applications, the more rapid developability and the ecological advantages of high chloride emulsions have provided an impetus for employing these emulsions over a broader range of photographic applications.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of selected tabular grain populations in photographic emulsions.
In almost every instance tabular grain emulsions have been formed by introducing two or more parallel twin planes into octahedral grains during their preparation. Regular octahedral grains are bounded by {111} crystal faces. The predominant feature of tabular grains formed by twinning are opposed parallel {111} major crystal faces. The major crystal faces have a threefold symmetry, typically appearing triangular or hexagonal.
The formation of tabular grain emulsions containing parallel twin planes is most easily accomplished in the preparation of silver bromide emulsions. The art has developed the capability of including photographically useful levels of iodide. The inclusion of high levels of chloride as opposed to bromide, alone or in combination with iodide, has been difficult. Silver chloride differs from silver bromide in exhibiting a much stronger propensity toward the formation of grains with faces lying in {100} crystallographic planes. To produce successfully a high chloride tabular grain emulsion by twinning, conditions must be found that favor both the formation of twin planes and {111} crystal faces. Further, after the emulsions has been formed, tabular grain morphological stabilization is required to avoid reversion of the grains to their favored more stable form exhibiting {100} crystal faces. When high chloride tabular grains having {111} major faces undergo morphological reversion to forms presenting {100} grain faces the tabular character of the grains is either significantly degraded or entirely destroyed and this results in the loss of the photographic advantages known to be provided by tabular grains.
Maskasky U.S. Pat. No. 4,400,463 was the first to prepare in the presence of an adsorbed grain growth modifier a high chloride emulsion containing tabular grains with parallel twin planes and {111} major crystal faces. The strategy was to use a particularly selected synthetic polymeric peptizer in combination with an adsorbed aminoazaindene, preferably adenine, acting as a grain growth modifier.
Maskasky U.S. Pat. No. 4,713,323 significantly advanced the state of the art by preparing high chloride emulsions containing tabular grains with parallel twin planes and {111} major crystal faces using an aminoazaindene grain growth modifier and a gelatino-peptizer containing up to 30 micromoles per gram of methionine. Since the methionine content of gelatino-peptizer, if objectionably high, can be readily reduced by treatment with a strong oxidizing agent U.S. Pat. No. 4,713,323 placed within reach of the art high chloride tabular grain emulsions with significant bromide and iodide inclusions.
Further investigations of grain growth modifiers capable of preparing high chloride emulsions of similar tabular grain content followed. As grain growth modifiers, Tufano et al U.S. Pat. No. 4,804,621 and Houle et al U.S. Pat. No. 5,035,992 employed 4,6-di(hydroamino)pyrimidines lacking a 5-position amino substituent (a 2-hydroaminoazine species); Japanese patent application 03/116,133, published May 17, 1991, employed adenine in the pH range from 4.5 to 8.5; Takada et al U.S. Pat. No. 4,783,398 employed heterocycles containing a divalent sulfur ring atom; Nishikawa et al U.S. Pat. No. 4,952,491 employed spectral sensitizing dyes an divalent sulfur atom containing heterocycles and acyclic compounds; and Ishiguro et al U.S. Pat. No. 4,983,508 employed organic bis-quaternary amine salts.
For a practical application silver halide emulsions must be chemically and spectrally sensitized. Sensitization of adenine-generated AgCl tabular grains is disclosed in Maskasky U.S. Pat. No. 4,400,463, where spectral sensitization is performed first in the presence of bromide ion followed by chemical, sulfur-plus-gold sensitization in the presence of thiocyanaye ion at 65.degree. C. Spectral sensitization of AgCl tabular grains by displacement of other than adenine growth modifier is disclosed in Tufano and Chan U.S. Pat. No. 4,804,621 (also U.S. Pat. No. 4,783,398 and U.S. Pat. No. 4,938,508). Further enhacements in spectral and chemical sensitization, and also adsorption of other photographically useful addenda, can be provided by by lowering the pH to protonate, and ultimately remove growth modifier from the crystal surface, as described in Maskasky U.S. Pat. Nos. 5,217,858 and 5,221,602. Specifically Jones and Osborne-Perry U.S. Pat. No. 5,176,991 describe a process of preparing an emulsion for photographic use comprising (1) forming an emulsion comprised of silver halide grains and a gelation-peptizer dispersing medium in which morphologically unstable tabular grains having {111} major faces account for greater than 50% of total grain projected area and contain at least 50 mole percent chloride, based on silver, the emulsion additionally containing at least one morphological stabilizer adsorbed to surfaces of the tabular grains, and (2) chemically sensitizing the tabular grains.
The process is characterized by the steps of choosing the morphological stabilizer from among 2-hydroaminoazines and xanthinoids, initiating protonation of the morphological stabilizer adsorbed to the tabular grain surfaces, performing the step of chemical sensitization while protonation of the morphological stabilizer is occurring, and terminating protonation of the morphological stabilizer so that at least a portion of the morphological stabilizer is retained on the surfaces of the chemically sensitized tabular grains.
As claimed in U.S. Pat. No. 5,176,991, by partially removing the morphological stabilizer during chemical sensitization greatly increased levels of photographic sensitivity can be achieved.